Companies need employees, but that doesn't mean they intend to go through the process of recruiting them by themselves. Sometimes they outsource this to agencies and recruiters. One of the two primary types of recruiters that offer these services is the contingency recruiter.
A contingency recruiter is an individual or agency hired by a company to headhunt potential candidates for a position. They are not part of the organization and yet do not receive an upfront fee for their services. Instead, they essentially research and present candidates to the company for free until a candidate of theirs is hired.
Due to the nature of the agreement that they have with companies, contingency recruiters don't necessarily get paid for every project. For starters, most companies that recruit this way use many such agencies and pit them against each other. This strategy widens the company's reach but can leave many agencies battling it out for the same payday.
It reduces the financial risk of a company. Contingency recruiters are called "no win, no fee" recruiters. This means that if they don't supply the winning candidate, they don't get paid. As such, companies bear no financial risk by contracting their recruitment to them.
Motivated approach. If a company hires the right contingency recruiter, it gets a motivated individual on its side. The competition brings out the best in contingency recruiters, motivating them to work faster and harder or risk losing out to other recruiters or agencies.
Inefficiency. Depending on the number of candidates submitted by the contingency recruiters doing the groundwork (usually a lot), hiring managers would have a lot of resumes to read. Compared to other types of recruiters, their use might be considered inefficient.
Lack of priority. Contingency recruiters sometimes juggle many projects at the same time. Since they don't have an exclusive contract with companies, they are not particularly inspired to give priority to all projects. A company's recruitment drive can find its mission at the bottom of a contingency recruiter's list of priorities.